Annealing ovens are used in a variety of commercial and industrial applications. In some of these applications, the annealing process includes imparting a magnetic orientation to the products or objects in the oven. For example, magnetic vacuum ovens have been used for magnetic annealing in the manufacture of devices such as read/write heads for rigid media storage devices, e.g., magnetic resistive (MR) and giant magnetic resistive (GMR) heads, disk drives, “MRAM” wafers, and the like. Such magnetic media are referred to as work pieces and are generally in the form of wafers or disks. Work pieces are typically formed of a substrate bearing magnetic film or layers to which a particular magnetic orientation has been imparted through exposure to magnetic fields at elevated temperatures. The process of imparting a particular magnetic orientation in this manner is known as annealing or magnetic annealing.
In the magnetic annealing process, the work pieces are heated to make them more susceptible to magnetic fields. The magnetic film or layers contain ferromagnetic material having a crystalline structure. Raising the temperature increases the vibrational moments of atoms forming the crystalline structure of the magnetic material and imparts a randomness to the motion of the atoms, weakening the crystalline structure of the ferromagnetic material in the magnetic film or layers. This places the atoms in a state that provides minimal resistance to the influence of an outside magnetic field. Exposing the heated wafers to a magnetic field causes the atoms to be held in place or oriented along the axis of the magnetic field. After subjecting the atoms or crystals of the magnetic media to elevated temperatures in the presence of a magnetic field of a desired strength for a prescribed period of time, the wafers are cooled, thus fixing or locking the atoms or crystals in the orientation imparted by the magnetic field. Thus, magnetic annealing involves both heating the media and subjecting the media to a magnetic field so as to orient the crystals of the magnetic film or layers thereof. This annealing operation is often performed with the magnetic media under vacuum conditions or in the presence of an inert atmosphere to minimize oxidation of the work pieces while they are at the elevated temperature.
The magnetic annealing process may be carried out on a single work piece or on multiple work pieces in batches or lots. The magnetic field can be generated by a permanent magnet, electromagnet or superconducting electro-magnet. Such magnets have been incorporated into vacuum ovens. However, permanent magnets are heat sensitive, losing their magnetism at temperatures above their Curie point, and hence should not be positioned too closely to sources of heat. Though permanent magnets are utilized, electromagnets and superconducting electro-magnets are particularly well suited for placement external to the chamber of a vacuum oven.